Published online 9 December 1999 | Nature | doi:10.1038/news991209-11

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Making sense of magma macinations

Some volcanic eruptions are gentle (as on Hawaii), others such as the eruptions of Mount St Helens in 1980 and Mount Pinatubo in 1991 are violent and terrifying, causing severe damage, death and even temporary climate change. Amongst the most deadly of volcanic phenomena are eruptions that give birth to 'pyroclastic flows', fast-moving flows of magma particles and gas. One such buried Pompeii at the foot of Vesuvius in AD 79. Understanding these explosive volcanic eruptions depends on knowledge of the break-up of bubbly magma, which transforms slow-moving lava streams into a swift and violent pyroclastic flow.

Now Youxue Zhang from the University of Michigan, Ann Arbor, USA, has identified a criterion for determining when a bubble-filled magma will fragment in a brittle cracking process. In the same issue of Nature [9 December 1999], David Dingwell of the University of Bayreuth, Germany, and colleagues report that porous pumice can provide a record of this transition from a ductile magma to a brittle medium on the verge of catastrophic break-up.

A brittle material cracks when it is pulled apart with a force that exceeds the material's so-called tensile strength. But it is hard to apply this simple idea to a bubbly magma, because the irregular distribution of bubble sizes and shapes causes significant local variations in both stress and strength.

Yet Zhang has now deduced how the presence of a gas-filled bubble in the magma modifies the criterion for brittle fracture around a single bubble. The behaviour of the whole magma body depends on the number, size and shape of such bubbles; but Zhang's new fracture equation isolates the basic physics of the break-up process, as well as showing how it depends on variables such as the water content of the magma.

Dingwell's group meanwhile suggests a kind of forensic approach to deducing retrospectively what happened during an explosive eruption that involves magma fragmentation. The bubbles are pulled and squeezed during this process, and may become frozen in these highly deformed shapes in so-called 'tube' or 'woody' pumice.

The researchers show that this texture gets fixed into the pumice at the moment of fragmentation, and so provides a record of the magnitude and direction of stresses it experienced at that stage. This might then help in reconstructing how the magma was flowing when the explosive eruption took place, and where and how this outburst started. 

  • References

    1. Zhang,Y. A criterion for the fragmentation of bubbly magma based on brittle failure theory Nature 402, 468 1999. | Article | ChemPort |